In a GSM system (Global System for Mobile Communications), a combination of frequency division multiple access (FDMA) and time division multiple access (TDMA) is used. The available frequency band is divided into an uplink band (890 MHz-915 MHz) and a downlink band (935 MHz-960 MHz) with a band spacing of 45 MHz when using a frequency division duplex (FDD) method. Each of these bands is subdivided into 124 individual frequency channels at a spacing of 200 kHz. Each frequency channel is unambiguously numbered and a pair of equal numbers from the uplink band and the downlink band forms one duplex channel with a fixed duplex spacing of 45 MHz. This is the FDMA component. Within each frequency channel, a TDMA method with 8 timeslots per timeslot frame is used, the timeslot frames of the uplink band being sent with three timeslots delay compared with the timeslot frames of the downlink band for reducing the switching effort. A subscriber station in each case uses the timeslot having the same timeslot number (TN) in the uplink band and in the downlink band. This correspondingly also applies to the expanded GSM frequency bands and for DCS (Digital Communication System) 1800.
In each timeslot of a timeslot frame, databursts of the same length are sent. A normal burst (NB) includes error-protection coded and encrypted user data, symmetrically separated by a so-called midamble (MA) for estimating the channel characteristics and corresponding channel equalization. The timeslot number, the midamble number and the channel type (control channel, traffic channel . . . ) apply both to the uplink band and to the downlink band in the GSM system.
In each timeslot of a timeslot frame, databursts of the same length are sent. A normal burst (NB) contains error-protection coded and encrypted user data, symmetrically separated by a so-called midamble (MA) for estimating the channel characteristics and corresponding channel equalization. The timeslot number, the midamble number and the channel type (control channel, traffic channel . . . ) apply both to the uplink band and to the downlink band in the GSM system.
This means that the logical or physical channel allocated to a subscriber station in a GSM system, is unambiguously specified in the uplink band and in the downlink band even without information on uplink or downlink, by means of a channel description.
If an optionally applicable frequency hopping is used during which the frequency is changed periodically during the transmission in order to compensate for frequency-selective disturbances, the frequency hopping parameters also apply to the uplink band and the downlink band.
In a DECT (Digital Enhanced Cordless Telephone) system which also uses a combination of FDMA and TDMA at the radio interface, the total available frequency band (between 1880 MHz and 1990 MHz) is used in both directions. In contrast to the GSM system, where transmission and reception takes place in different timeslots for separating uplink from downlink. This is called a TDD (time division duplex) mode. According to the DECT standard, the first 12 timeslots of a DECT frame are provided for the downlink and the second 12 timeslots of the DECT frame are provided for the uplink. There is always a spacing of 12 timeslots between uplink and downlink of a voice connection. These 12 timeslots correspond to a period of 5 ms because the DECT system operates with a fixed switching point between downlink and uplink. If a DECT subscriber station requests a voice channel (full slot) on a particular timeslot, for example timeslot 18, and on a particular frequency fx, the uplink channel is unambiguously specified in accordance with the DECT standard. The uplink channel is on the same frequency fx and on timeslot 6 (18-12).
Future radio communications systems such as UMTS (Universal Mobile Telecommunication System) which, among other things, will offer a transmission capacity comparable to ISDN for services, such as video telephony and broadband connections, and will be used in the text which follows for representing the technical background of the invention without restricting the general applicability of the use of the invention, are based on the transmission channels being separated by spread-spectrum codes. The significant feature of a code division multiple access (CDMA) method is the transmission of a narrow-band radio signal in a wide frequency spectrum, the narrow-band signal being spread to a wideband signal by means of a suitable coding rule. In the UMTS system, two modes are provided, the FDD mode and the TDD mode. The FDD mode is a broadband CDMA characterized by the degrees of freedom of frequency and spread-spectrum code and the TDD mode is a TD/CDMA method characterized by the degrees of freedom of frequency, timeslot and spread-spectrum code. In the latter, the multiple access is achieved by means of a broadband TDMA/FDMA system in which a multiple access according to the CDMA method is allowed in certain timeslots of a timeslot frame. In the TDD mode, one or more variable switching points between uplink and downlink are provided within a timeslot frame, in order to achieve better management of the scarce frequency resources.
In the UMTS system, different frequency bands are provided, unpaired bands and paired bands. From current perspective, the unpaired bands are reserved for the TDD mode and the paired bands are exclusively reserved for the FDD mode. One unpaired band is in the frequency range of 1900 MHz to 1920 MHz and the other unpaired band is in the frequency range of 2010 MHz to 2025 MHz. The uplink band of the paired band is in the frequency range of 1920 MHz to 1980 MHz and the downlink band of the paired band is in the range of 2110 MHz to 2170 MHz. The duplex band thus has a duplex spacing of 190 MHz. The frequency bands are divided into frequencies of 5-MHz bandwidth each. The unpaired bands thus have four and three frequencies and the paired band has 12 uplink frequencies and 12 downlink frequencies. FIG. 3 provides a representation of the frequency bands and how they are divided up.
When symmetric services are requested such as, for example, services with data rates of 64 kBit/s, 144 kbit/s or higher (real-time service) or also voice services, the same data rates must be transmitted in the downlink band as in the uplink band.
In the case of asymmetric utilization of the paired band, the downlink band is heavily loaded and the uplink band is loaded only slightly. This can be expected, in particular, in the case of database enquiries such as from the Internet. In the case of asymmetric data services, it is assumed that a high data rate is required in the downlink and a low data rate in the uplink. Naturally, the situation can also occur the other way around, for example when sending a fax from a subscriber station.
For this purpose, it has been proposed to allow a TDD mode in the uplink band of the paired band from the UMTS as a result of which a higher capacity utilization of the frequency resources is supposed to be achievable overall. This requires a new protocol for an unambiguous channel description which must be implemented both in the subscriber stations and in the base stations.
The invention performs an efficient description of the channels with little signaling expenditure.